For several years, technologies in the field of regenerative medicine have focussed on stem cells notably pluripotent stem cells (such as embryonic stem cells and more recently induced pluripotent stem cells), as these cells have the capacity to self-renew and to differentiate into multiple specialized cell types. The concept of regenerative medicine involves transplanting cells of interest with the goal of repairing and regenerating a target tissue and/or target organ which cannot regenerate itself, since most tissues or organs, such as heart tissue and neural tissue, are essentially composed of functionally differentiated somatic cells and cannot regenerate alone or, at least, cannot regenerate efficiently, due to their very limited capacity of self-renewal.
Indeed, in metazoan organisms terminal differentiation is generally tightly linked to cell cycle exit, whereas the undifferentiated state of pluripotent stem cells is associated with unlimited self-renewal. The non-proliferative state of terminally differentiated cells is notably assured by robust, often redundant mechanisms and in rare exceptions where fully mature cells can re-enter the cycle, proliferation remains transient and usually involves de-differentiation. It remains unknown what renders differentiated cells refractory to the very mitogen signals that stimulate the proliferation of their direct precursors. For example, the proliferative response of myelo-monocytic progenitors to M-CSF is lost upon differentiation to macrophages, despite the continued ability of these mature cells to sense the cytokine. Consequently, myeloid progenitor cells form colonies in semi-solid M-CSF containing medium, whereas blood monocytes and tissue macrophages do not.
However, international patent application WO 2008/084069 recently discloses a method for generating, maintaining and expanding monocytes and macrophages in long term culture by inhibiting the expression or the activity of MafB and c-Maf in said cells; and expanding the cells in the presence of at least one cytokine, such as M-CSF.
Now, the inventors have underlined that such method is based on a mechanism that depends on regulated activation of c-Myc and Klf4 and have surprisingly demonstrated that long term proliferating cells thus obtained are not tumorigenic despite the fact that c-Myc and Klf4 are both oncogenes as described in Rowland et al. 2006 and Adhikary et al. 2005.